1. Field of the Invention
The present invention relates to hot melt ink used for an ink jet printer, and more particularly, to hot melt ink which is solid at room temperature, and melts at high temperature for making a print image at the time of printing.
2. Description of Related Art
An ink jet printer is conventionally proposed for recording desired characters and images on a recording medium such as paper by directly placing an ink droplet jetted from a print head. This ink jet printer has a lot of advantages in the print techniques such as no noise, a high-speed print, a high-quality print, and a color print. However, in the above ink jet printer, the jetted ink droplet is directly placed on the recording paper, so that a print quality or time for drying a printed image are changed according to the kind of the recording paper. That is, there is a problem that the print quality is remarkably deteriorated when it is printed on low quality recording paper, the printed image being blurred and illegible.
In order to solve such a problem, it has been proposed to use ink of a hot melt type as ink for the ink jet printer. The ink of the hot melt type has a melting point higher than the room temperature. It is well-known that a print image could be printed on all kinds of recording paper with the above ink, because the ink becomes solid with a uniform dot diameter on all kinds of recording paper. Various compositions are known for such hot melt ink. For instance, one hot melt ink which contains natural wax is disclosed in U.S. Pat. No. 4,390,369. Further, another hot melt ink which contains stearin acid is disclosed in Japanese Laid-Open Patent Publication No. 59-22973. A third hot melt ink which contains acid of C.sub.20 -C.sub.24, alcohol of C.sub.20 -C.sub.24, or ketone whose melting point is 65.degree.-120.degree. C. are disclosed in U.S. Pat. No. 4,659,383. In the above mentioned ink, a dye is used as a color material. Moreover, a forth hot melt ink is disclosed in U.S. Pat. No. 4,659,383. In such an ink, the solid pigment as a color material is dispersed in one of wax, fatty acid of C.sub.18 -C.sub.24, or alcohol of C.sub.18 -C.sub.24. The wax, fatty acid, and alcohol have melting points which are higher than 65.degree. C. respectively.
However, as mentioned above, the hot melt ink using the dye as the color material is not always superior in its dye solubility. Thus, there is a problem in that the kind of dye to be used is limited. Further, the hot melt ink does not have a characteristic which is always steady during heating for a long period of time or during the repetition of heating and cooling. Also, there is a problem in color fastness to light. Moreover, in the conventional hot melt ink using the dye or the solid pigment as the color material, there is a problem that the size of a recorded dot formed by an ink droplet on a recording paper is decided by an ink characteristic and a device characteristic, so that there are limitations to increases in the density of the recorded dot. The ink characteristic is a characteristic such as the viscosity and the surface tension of ink, and the device characteristic is a characteristic such as the jet nozzle diameter, the jet power, and the jet temperature of the ink jet device.